The invention relates to a method for determining the position of at least two seismic streamers in a reflection seismic measuring system, in connection with marine seismic surveys.
Lately there have come into use three-dimensional exploration methods for marine seismic surveys. Such exploration methods place heavy demands on navigation and positioning and one is dependent on knowing the relative positions of seismic sources and the hydrophones of the seismic streamer with great precision. In modern seismic exploration methods several sources and several seismic streamers are usually employed, and the mutual distances between these must also be determined with relatively high precision. The seismic sources and the seismic streamers are usually towed by one or more exploration vessels, and the exploration vessels' absolute position at any time is determined by means of surface navigation systems aboard the vessel or the vessels, these navigation systems preferably being land-based or satellite-based radio navigational systems which give a resolution below 0.5 m and a repeatable positioning accuracy of a few meters. It is then necessary to determine the position of the seismic streamers or in reality the position of the hydrophones of the seismic streamers with as high accuracy as possible, the basis for this position determination being the absolute position found by the navigational system of the vessel.
A seismic streamer comprises a plurality of active streamer sections with known length and equipped with hydrophones or hydrophone groups with known location in the active sections. The seismic streamer is arranged between stretch sections, a fore stretch section being connected with a towing means on the towing vessel, whereas a tailbuoy with means for determining the position is provided at the end of the aft strech section. The means may for instance be an active navigation system of the same type that is employed aboard the towing vessel or a microwave system, possibly combined with a goniometer. The position of the tailbuoy may also be determined by passive distance measurements between the towing vessel and the buoy, for instance by means of radar or laser reflectors. Now knowing the positions of the vessel and the tailbuoy, the position of the hydrophones in the seismic streamer is determined on the basis of the known length of the streamer, the known location of the hydrophones in the streamer and the orientation of the separate sections of the vessel and the tailbuoy. This orientation may in principle be provided by taking a bearing between the vessel and the tailbuoy using compass devices aboard the vessel. However, due to swing of the seismic streamer caused by sea currents, an angular deviation between the streamer and the ship's bearing is generated. In practice the seismic streamer will hence have the shape of a plane or a spatial curve, but the said angular deviation may be determined by providing a plurality of magnetic compasses in the cable, typically for instance twelve compasses in a streamer of three kilometers length, and normally with a compass close to each end of the streamer. Compasses are also provided in the stretch section. The curve of the streamer may then be determined for instance by means of mathematical estimation in connection with compass bearings, known distances and section lengths. As a rule, however, the error in the actual distance to a streamer section will lie within the errors of the position determining system, referred to the navigation accuracy.
At present a number of methods are employed or proposed to be employed, for determining the streamer's position, based on distance measurements in connection with compass bearings. One method is to determine supposed distances to the single streamer sections based on how much towing cable is being handed out from the towing vessel, at the same time as bearings are taken with the magnetic compasses in the streamer. The method, however, is encumbered with substantial errors, since the stretch sections of the streamer may be stretched such that the distances may vary with about 10 to 15 m, and there are small possibilities of determining this discrepancy accurately. Changes in the course of the vessel will result in a poorer determination of bearing when the streamer with compasses starts to stretch as a result of the movement of the vessel.
Norwegian patent application 83 1513 discloses a method for determining the position of a seismic streamer which is towed through the sea by a vessel. Herein azimuth and distance from the vessel to points on the seismic streamer are measured, and the coordinates of the points are calculated by means of these values. Furthermore, a hydroacoustic measurement method based on a ultra-short base line system is used, which is integrated with the gyro-compass of the vessel for azimuth and distance measurements against transponders, responders or similar devices provided on or in the seismic streamer. This method gives no measurement of redundancy and there are hence limited possibilities of discovering errors in the system. Further, one is dependent on determining a reference bearing and this bearing must be taken with a gyro-compass having a limited accuracy.
Another method which uses acoustic distance measurement techniques for determining the streamer position is described in the paper "Improving the accuracy of marine 3-D seismic surveys", Ocean Industry, Jan. 1987. Here acoustic transceivers provided on the fore end of the seismic streamer and at the seismic sources are used, while acoustic receivers are provided aboard the vessel. The determination of directions are made by compass bearings, but relatively acute intersecting angles give fairly large errors in the angle readings and in addition too few measurements for achieving sufficient redundancy and determination of measurement errors. A further hydroacoustic positioning system for two seismic streamers is disclosed by a report from Sonardyne Ltd. with the title "A hydroacoustic system for precision tracking of twin hydrophonic streamers" (ref.: C/87/363). This system provides more measurements so as to achieve a somewhat better redundancy than the aforementioned system. The determination of the direction takes place by compass bearings, but the bearings of the seismic streamer fore ends result in relatively acute angles having fairly large measurement errors. Additionally, the position of the seismic source is not determined.
An acoustic distance measurement technique has also been applied to the problem of determining the horizontal profile of a towed seismic streamer. To that end US-PS No. 4 532 617 (Baecker and Bijou) discloses a system and a method based on using a slave vessel in addition to the towing vessel, acoustic transponders being provided in the vessels and along the seismic streamer. The positions of the vessels are determined and the distances between the vertices of a triangle formed by the vessels and a respective transponder on the streamers are found, allowing a representation of the profile of the streamer to be obtained. The method as taught by the publication thus employs per se known techniques, but the proposed system is not well suited for determining the position of several towed objects and would be hightly impractical to adopt for positioning tasks in a 3-D marine seismic surveying system employing more than one streamer.
Systems wherein distance measurements are made by means of optical methods and microwave methods are also encumbered with a number of deficiencies. Using a laser one is dependent on optical visibility and both laser measurements and radiogoniometry must be used in connection with compass bearings, which in view of the measuring errors of the gyrocompass leads to inaccurate direction determinations. Furthermore, it is only possible to determine the position of floats, rafts, paravanes etc., i.e. devices which float on the surface of the sea. Thus, all the above-mentioned methods have certain disadvantages and deficiencies. Even if distances and absolute positions may be found with sufficient accuracy, these disadvantages, however, are of such a nature that the methods do not furnish a general, total measuring accuracy or sufficient redundancy to achieve an accurate determination of the measuring errors.